The way live traffic is currently filtered, only about 20% (at the very most) of actual real world traffic is represented in the sim. It’s something that Asobo are aware of and something that’s planned to be fixed in a future world update.
That said, the amount of traffic you see is very inconsistent. I can fly across Florida (busy airspace) and see a whole bunch of planes. Then I can go to KATL, the busiest airport in the world and see only a few planes. It’s really hit or miss when it comes to live traffic.